1. Technical Field
The present invention relates generally to ion implantation, and more particularly, to a system, method and program product for monitoring and adjusting beam angle integrity of an ion beam.
2. Related Art
One of the most important steps in the fabrication of semiconductors is doping the materials used to make the semiconductor. Doping is a process in which atoms of an element are introduced into a semiconductor for the purpose of altering its electrical properties. This may be accomplished by in situ doping, in which the atoms are introduced during the growth of the crystal, or, more often, may be introduced after crystal formation in a process known as ion implantation. In this process, ions of the desired dopant are implanted into the surface of the target substance such as silicon.
One of the systems that is most widely used for ion implantation type doping is an ion accelerator type ion implanter system. A typical ion accelerator type ion implanter system has an ion generator that generates ion particles. The system then uses magnets to accelerate the ion particles, focus the ion particles into an ion beam and direct the ions in the ion beam to the target into which they are to be implanted.
One problem in the industry lies in monitoring the integrity of the ion beam generated by the ion accelerator type ion implanter. A discussion of the ion beam angle integrity problem is located in U. Jeong, S. Mehta, C. Campbell, R. Lindberg, Z. Zhao, B. Cusson and J. Buller, “Effects of Beam Incident Angle Control on MNOS Source/Drain Extension Application,” 14th International Conference on Ion Implantation Technology, Taos, NM, USA, Sep. 22-27, 2002, which is hereby incorporated by reference. In an optimal ion beam, all ions travel along an ideal path or trajectory. Because ions of the same charge are repelled from one another, some variance of ions from the ideal path of the ion beam is normal. However, variance beyond a certain angle from the optimum trajectory can be problematic. This variance may be caused by collisions among ions in the beam or by the above mentioned natural repulsion of the ions from one another leading to portions of the beam becoming divergent (beam non-parallelism). Additionally, the ion implanter system may, over time, become misaligned, causing the entire beam to deviate from the normal angle (beam steering). FIGS. 1A and 1B are cross-sectional views of ions that deviate from a normal angle striking targets. Both of the targets in FIGS. 1A and 1B have alternating substrate layers, labeled silicon nitride (“nitride”) and silicon dioxide (“oxide”). The target in FIG. 1A is said to be an angle insensitive structure because even though beam steering has occurred, the relative width of the trench ensures that a large percentage of the ions reach the bottom of the trench. Conversely, the beam steering combined with the relative height of the trench in FIG. 1B causes a relatively small percentage of ions to reach the bottom of the trench. The target in FIG. 1B is referred to as an angle sensitive structure because it is particularly sensitive to any deviation in the angle of the ion beam. Additionally, a small undercut P1 created by isotropic overetch causes further sensitivity of the FIG. 1B target to ion beam angle deviation.
Ions that strike a target from a trajectory outside of the maximum variance angle may cause unwanted changes, known as shadowing, in the material that the ions strike that is adjacent to the desired implant area. This shadowing may cause the material in the device to function other than optimally. For example, a large number of ions striking outside the trajectory of the maximum variance angle may cause the device to function incorrectly or not at all. Accordingly, it is important to be able to monitor the beam angle integrity of the ion implant beam.
In view of the foregoing, there is a need in the art for monitoring the integrity of the beam angle of an ion implant beam.